1. Field of the Invention
The invention described herein relates to accumulating wave energy, and in particular, to a method and apparatus for converting energy in water waves into potential energy for performing useful work and/or directly into useful work.
2. Description of the Related Art
Electrical energy for general use is being generated primarily by natural water flow, by burning fossil fuels and by nuclear generation. These methods have associated environmental problems and in the case of fossil fuels, an unacceptable depletion rate. This has led researchers to investigate the possibilities of harnessing energy from wave action and from wind.
A great variety of approaches have been proposed to generate energy from waves. One common approach is to utilize a float which rides up and down with wave motion guided by a fixed or anchored member which remains relatively stationary. An air compression cylinder is introduced between the float and the anchored member, together with associated intake and output conduits and check valves in order to store energy in compressed air which is accumulated as the float rises and falls with the wave action. Variations to such devices have been made to pressurize liquids or to pump water to fill an elevated water reservoir on shore. The potential energy stored in the pressurized fluid, e.g. air or water, is then used to drive a conventional machine such as a turbine/generator set to supply electrical energy.
One example of a prior art approach to the use of such arrangements is found in U.S. Pat. No. 644,093 to Place (issued Feb. 29, 1900). This older patent describes a conventional marine air compressor which also includes a submerged reservoir. The submerged reservoir is positioned below the float and air compression cylinder, and remains relatively stationary in deeper waters away from the active wave surface. The reservoir is anchored to the ocean floor and is connected in line between the air compression cylinder and the compressed air output conduit. A fluid filled dampening piston-cylinder arrangement is provided within the reservoir to compensate for the rise and fall of the water surface between high and low tide in ocean applications.
Another example is found in U.S. Pat. No. 4,754,157 to Windle (issued Jun. 28, 1988) which describes several further variations upon conventional float type wave energy devices, including means to extract energy from both the rise and fall of the float. Windle also teaches the use of a number of such devices connected in parallel to fill a water reservoir.
Single float type units are also described in U.S. Pat. No. 1,665,140 to Master, 2,487,228 to Kriegel, U.S. Pat. No. 3,515,889 to Kammier, U.S. Pat. No. 4,203,294 to Budal et al and U.S. Pat. No. 4,560,884 to Whittecar. Multiple parallel arrays of such units are described in U.S. Pat. No. 1,264,737 to Woods, U.S. Pat. No. 4,204,406 to Hopfe, U.S. Pat. No. 4,408,454 to Hagen et al, and U.S. Pat. No. 4,622,473 to Curry.
Conventional single unit devices and multiple arrays of units conventionally connected in parallel suffer from the disadvantage that a relatively high minimum amplitude of wave must be encountered before the pressure in the cylinder reaches a level at which useable pressurized fluid is generated. As a result, waves having an amplitude below such a minimum do not generate any energy. The minimum amplitude is determined by the design of the air compressing cylinder, and this disadvantage is present in both single unit devices and multiple unit arrays of the conventional methods and devices.
To clearly illustrate this point the following example is presented.
Assuming:intake pressure Pi = 14 psiaintended design output pressure Pf = 250 psiaair cylinder internal diameter d = 10 inair cylinder intake chamber length Ii = 60 ininitial volume Vi = ¼ π d2 Iifinal volume Vf = Vi Pi/Pf = ¼.π d2 Ifsince PV = constant at a constant temperaturesolving for If = Ii Pi/Pf = 3.36 inches
In summary, before compressed gas of 250 psia opens the check valve and exits the chamber, the piston must move over 56 inches or 95% of its full stroke. From the above example, it can be seen that the larger the pressure difference Pd between intake pressure Pi and output pressure Pf the greater will be the length or percentage of the stroke required, relative to the cylinder length, before any compressed gas leaves the device (as represented mathematically If=Ii Pi/Pf).
As in the above example, the outlet of a chamber has a check valve which will only open to exhaust compressed gas when the pressure of the gas is equal to or greater than 250 psia. If a wave is encountered which provides a stroke of less than 56.64 inches, the gas is pressurized to a level less than 250 psia and there is no output of compressed gas. In effect the potential energy stored in the compressed gas generated at less than the final design pressure is not captured because the outlet valve does not open. The potential energy of the compressed gas below 250 psia is dissipated as the wave subsides and the gas decompresses.
Lowering the design pressure will increase the volume of compressed air generated and will capture energy from waves of lesser amplitude (i.e. if Pf=125 psia then If=6.72 inches; if Pf=50 psia then If=16.8 inches). This design choice is of little practical value because with compressed gases of lower pressure, higher volumes must be conducted and low pressure gases are of lesser value in driving turbines. Also, the decrease in pressure output is not offset by the decrease in minimum wave amplitude (i.e. from Pf=250 to 50 psia the minimum wave amplitude required to effect a stroke of 56.64 inches to one that effects a stroke of 43.20 inches is only 13.44 inches or 24%).
The conventional parallel arrays of compression units do not address this disadvantage and the total volume of energy captured is increased only by increasing the number of compression units. This solution is not cost effective given the capital cost of building an array of units and the operational cost of maintaining them.
Applicant's U.S. Pat. Nos. 5,179,837 and 5,394,695 disclose a method and system for generating energy from the motion of waves by a plurality of series-connected floating pressurization units, wherein each unit incrementally increases the pressure in a compression fluid flowing therethrough using energy in water waves. The system provides for a new ambient pressure starting point at each stage in the series, such that each successive downstream stage has a higher initial pressure than the previous stage(s). Energy is transferred from the waves to the fluid by relative movement between a first floating portion and second submerged portion of the unit. The second portion has a piston that extends upwardly into a compression chamber of the first portion. Vertical oscillation of the first portion as it reacts to the wave causes the piston and chamber to vertically move relative to each other, thereby compressing the fluid in the chamber and driving the fluid through an outlet valve and to the next adjacent pressurization unit. When the cylinder bottom moves away from the piston, fluid compression stops and new fluid flows into the cylinder. The outlet valve is biased to open at a selected outlet pressure; when the pressure in the chamber decreases below the outlet pressure, fluid transmission to the next pressurization unit stops. Therefore, fluid tends to be conveyed through the device in pressurized pulses, wherein the oscillation of the pulses depends on the oscillation of the waves.
While applicant's device in his previous two patents disclose a useful means for storing wave energy in the compression fluid, it is desirable to refine and improve upon the ideas introduced in these patents. For example, it is desirable to smooth out the delivery of pressurized fluid through the device.